The present invention relates to a device for measuring web tension of a continually transported material web.
The measuring device of the invention is particularly suitable for use in roller rotary printing machines in which paper web is continually moved and its movement is recorded. Automatic control, regulating, examining or scanning devices mounted in connection with the web transporting device are normally required for this purpose.
It has been known that web tension is normally measured directly or indirectly by electric means, and the devices for measuring web tension are engaged with a movable element applied to the outer face of the transported web. The devices measure a path deflection of the web wrapped about the roller which is a so-called compensating or oscillating roller. Such a device has been disclosed, for example in DE-PS No. 2,356,009.
The chief disadvantage of the known devices resides in that the functional position of the rollers changes, and relatively great changes in the lengths of the web paths between two printing devices, for example printing mechanisms, result. Register errors that occur in the roller rotary printing machines are too great for register control devices. A further disadvantage of the known measuring device is its relatively great inertia which prevents high frequency web tension changes, which are connected with web breaks occurring with alternating amplitude, from being detected.
Disadvantageous also is backfeeding of this known measuring device as well as costs of roller lever means causing backfeeding (weight, springs) and means for additional loading and unloading.
Furthermore there are known measuring devices in which web tension is detected by means of force-value detecting gages positioned at two sides of the web guiding roller, these gages being semi-conductor strain gages. Such a measuring device has been disclosed in DE-PS No. 2,211,598. A bending torque exerted on the roller due to the web tension and web wrapping is used and pressure and pulling forces resulting from this torque in both bearings are measured. An amount of an elastic deformation of a transverse beam or a bearing element, which occurs due to a position change of the roller, is measured by strain gages attached to the transverse beam or the bearing element, respectively.
The disadvantage of the conventional force-measuring devices is that these devices are very expensive because they include costly electronic equipment, power supply devices, aligning or adjusting apparatus, measurement transformers, loading arrangements provided for example with Wheatstone bridges and strain gages glued to steel members, these loading arrangements leading to high failure rate caused by the loading of the glue connections.
A further disadvantage of the force-measuring conventional device resides in the necessity of the measurement of forces in the two bearings of the web guiding roller. This method requires two measuring pick-ups and additionally an electronic device for the summation or the formation of a medium value resulting from the measurements in two roller bearings.
Also known are devices for measuring web tension in which elastic deformations of the connection means, for example supporting levers of the register roller pairs provided in the printing machine, are measured. Such a device has been disclosed in DD-PS No. 71 772. By means of a respective design of the connection means an extremely small but sensitive measuring device detects elastic deformations. For rigidly supported roller arrangements, for example web-guiding rollers, it has been suggested to measure elastic deformations of the elements supporting the roller bearing arrangement in the machine frame. The disadvantage of this method resides in that the bending torque on the roller should be measured and two measuring devices for the measurements in two roller bearings should be provided.